The present invention relates generally to improved motor protectors and more specifically to motor protectors particularly useful with split phase motors commonly used with refrigerator compressors and the like.
Over the years there have been a number of different ways used to provide thermal protection for motors to prevent fault conditions from causing overheating of the motors with concommitant burn out or other damage to the motor. For example, it is known to provide thermal protection by placing a motor protector switch on or in the motor windings in order to enable the protector to closely follow the temperature of the motor and to be sensitive to overtemperature conditions so that it can be deenergized before any damage due to overheating occurs. See for example United Kingdom patent specification No. 1,096,511 in which a temperature sensor is placed in the motor winding. However when used with refrigeration compressors or the like, special protector constructions are required in order to provide a hermetic seal so that the switch environment is not affected by the refrigerant fluid. Such seals add significantly to the cost of the device.
Yet another approach is to place the protector so that it is in close spatial relationship with the compressor casing to thereby sense the temperature of the casing to use the casing as a thermal analog of the motor winding temperature. This can be seen in German patent application DE No. 31 18 638 Al, and Japanese Utility Model No. 57-226. However since the casing is electrically grounded, electrical insulation is required between the casing and the protector, for example a layer of air or other electrical insulation, which also tends to make the protector less sensitive to the temperature of the casing. Further, due to the thermal mass of the casing there is an undesirable inherent time lag between the motor winding temperature and the casing temperature which adversely affects the tracking ability of the protector relative to the windings.